This invention relates generally to integrated circuits, and more specifically, to switching amplifiers for processing audio signals.
Audio switching power amplifiers are well known and widely used. Such amplifiers receive an audio signal that has been modulated, such as a pulse modulated digital signal. Most high efficiency digital audio switching power amplifiers are based on Pulse-width modulation (PWM). PWM is widely used in a variety of applications such as digital audio amplifiers and control applications including motor controllers. Many of these applications convert a sampled digital signal to a digital pulse-width modulation signal in order to obtain high efficiency and high accuracy. The PWM signal is presented to a switching amplifier that performs a level shifting function to translate the digital PWM input signal to a digital PWM signal having significantly higher voltage levels. To accomplish the power amplification, a higher voltage power supply is used.
A class of switching amplifiers is known as class AD. Class AD is a modulation form where only two discrete levels are used, e.g., 1 and xe2x88x921. In a class AD modulated amplifier, either plus the power supply voltage or minus the power supply voltage is applied to the load.
A source of noise in switching amplifiers is associated with the frequency at which the output signal is switched when being amplified. A noise component exists at the switching frequency and at harmonics thereof. Although the noise in the frequency spectrum at the switching frequency and its harmonics is typically out of the passband, such noise nonetheless creates standby power loss due to circulating currents and thereby reduces efficiency. In order to remove these undesired frequency components, a filter is usually required to eliminate the switching or carrier frequency. For typical systems, a four-pole L-C filter is necessary to sufficiently suppress the switching frequency. Because of its size and required inductive elements, the filter is one part of the system that is not suitable for integration. Additionally, the cost of such a filter is significant and typically eliminates such switching amplifiers from low cost high volume applications.